1. Field of the Invention
The present invention relates to the oxidation of polyacrylonitrile or other carbonizable material, and more particularly to oxidation processes in which an elongated web comprised of tows of the carbonizable material is continuously pulled through an oven under tension to accomplish oxidation thereof.
2. History of the Prior Art
It is known to carbonize polyacrylonitrile (PAN) or other carbonizable material by first oxidizing the material and thereafter heating the material in an inert atmosphere to a temperature sufficient to substantially carbonize the material. In the case of PAN, such material is often processed in the form of an elongated web of tows which are disposed in generally parallel, side-by-side fashion along the length of the web formed thereby. The web is fed under tension through relatively complex paths defined by rollers within one or more oxidizing ovens prior to being introduced into a carbonization furnace. Within the oxidizing oven the web makes multiple passes through different stages of the oven maintained at temperatures designed to achieve the desired oxidation of the PAN tows.
The nature of the PAN tows is such that the web cannot be allowed to remain at rest but must be kept continuously moving through the oxidizing oven when the oven is at oxidizing temperatures. To allow the web to remain at rest for any period of time would permit rapid deterioration and possible exotherming of the PAN tows. Moreover, even when the web is kept continuously moving through the oven, there cannot be any loose ends or knots in the tows. If loose ends or knots are present, they will usually exotherm in the hot oven resulting in interruption of the process and frequently the need to shut down the entire process and again introduce the web into the oxidation oven when cool.
For this reason the web of PAN tows is typically introduced into the oxidation oven when the oven is cool. Introduction is accomplished by tying the individual tows to various locations along the length of a threader bar having cables attached to the opposite ends thereof, The cables are used to pull the threader bar and attached tows into and through the oxidizing oven. Because the oven is cool, the knots and loose ends of the tows where they are tied to the threader bar do not exotherm.
When the tows have been pulled completely through the oxidizing oven by the threader bar, the oven is turned on and is heated up to oxidizing temperatures as the web of tows continues to be fed therethrough. The oven normally requires approximately two hours to reach oxidizing temperatures, during which time the web is continuously pulled through the oven. When the oven reaches oxidizing temperatures new portions of the web entering and pulled through the oven are oxidized in the desired manner. The preceding portions of the web which typically comprise about 200 to 400 pounds of PAN tows must be discarded as wastage. Because the PAN tows are relatively expensive, this represents a significant economic disadvantage in terms of the economics of the overall process.
It is seldom practical or for that matter possible to keep processes of this type running on an indefinite basis because of the need to keep personnel present at all times on a round-the-clock basis. In addition, power failures and other interruptions usually result in the need to let the oxidizing oven cool off, following which the startup process described above must be repeated. Accordingly, wastage of substantial quantities of PAN tows has become a routine and necessary part of carrying out such processes.
Because of the need to have a web of material under appropriate tension present within the oxidizing oven in preparation for the oxidation of fresh PAN tows, one technique commonly employed for shutting down the process is to continue running the web through the oven after the oven is turned off until the oven has cooled down sufficiently so that the web can be brought to rest. Upon subsequent startup, movement of the web through the oven is begun as the oven is heated up to oxidizing temperatures. The portions of the web which are run through the oven during the cooling off of the oven and the subsequent turning on thereof must be discarded as wastage.
In some cases the individual tows of the web are severed at the entrance to the oven after the oven has cooled down and the passage of the web therethrough stopped. When this occurs the process may be started by securing the web to be oxidized to the PAN tows residing within the oven. In such instances the individual tows of the web to be introduced into the oven are tied to the individual tows of the web residing within the oven, following which advancement of the webs through the oven is begun. To prevent exotherming of the knots and loose ends where the tows are tied together, the oven is turned on in stages with each stage being turned on after the ties in the tows have cleared that stage. Again, substantial amounts of the PAN tows are wasted before the oven can be brought up to operating temperature so as to begin oxidizing the web in the desired fashion.
Where the web comprises PAN fabric rather than individual tows of PAN material, lengths of the fabric have been joined together using various techniques such as that shown by way of example in U.S. Pat. No. 4,077,822 of Logwin.
It would therefore be advantageous to provide a method of introducing a web of carbonizable tows into an oxidizing oven in such fashion that wastage of the tows is minimized.